(1). Field of Invention
The present invention relates to a process for preparing pentachloronitrobenzene. More particularly the invention relates to a process for producing pentachloronitrobenzene having unexpectedly reduced levels of hexachlorobenzene while simultaneously avoiding excess contamination with pentachlorobenzene.
(2). Prior Art
Pentachloronitrobenzene is a commercially important fungicide for application to soil and seeds for controlling various plant diseases. It is particularly effective in controlling plant diseases caused by botrytis, fusarium, rhizoctonia and anthracnase.
Several methods are known for preparation of this compound. Those of significance to the process described herein involve nitration of pentachlorobenzene with mixed nitration acid which, as used herein, consists essentially of a mixture of nitric and sulfuric acids.
Canadian Patent No. 620,338 discloses one such process wherein 50-90% of the pentachlorobenzene is added to preheated ((80.degree. C.- 90.degree. C.) mixed nitration acid while maintaining the resulting temperature in the range of 90.degree. C. to 95.degree. C. The remaining pentachloronitrobenzene is then added while maintaining a temperature of 90.degree. C.-100.degree. C. following which the temperature is raised and held at 120.degree. C. to 130.degree. C. to complete the reaction. The reaction mixture is then cooled and product recovered. As best seen from Example 3 of the patent, the resulting product (after removal of sulfuric acid, nitric acid and water) contains about 1% pentachlorobenzene and about 2.1% hexachlorobenzene. Industry standards require that these impurity levels be reduced substantially. This is clearly not attained utilizing the temperature control scheme set forth in this patent.
Pentachloronitrobenzene has been prepared commercially utilizing mixed nitration acid by adding excess mixed nitration acid to pentachlorobenzene at a rate sufficient to maintain a reaction temperature in the range of 120.degree. C.-135.degree. C. The resulting product at this point in the process is a slurry of undesirably fine pentachloronitrobenzene crystals which may contain unacceptable levels of pentachlorobenzene. To improve crystal characteristics and deplete pentachlorobenzene, the slurry is then rapidly heated to above the melt point (142.degree. C.-144.degree. C.) of the pentachloronitrobenzene, and then cooled to recrystallize pentachloronitrobenzene. While the pentachloronitrobenzene thus produced contains minimal amounts of pentachlorobenzene, it still contains about 1-1.5% hexachlorobenzene, a commercially unacceptable level. This process has also been practiced using an initial reaction temperature of about 105.degree. C.-110.degree. C. with a rapid heat up cycle following acid addition as in the commercial process. Little, if any, reduction in hexachlorobenzene content was noted. The reaction has also been run at an addition temperature of 140.degree. C.-145.degree. C. producing a pentachloronitrobenzene product containing from 1-1.9 percent hexachlorobenzene. Jackson et. al., J. Org. Chem. 36 (23) 3638-9 (1971).
Considerable research effort has ben expended to find means for nitrating pentachlorobenzene with mixed nitration acid to produce pentachloronitrobenzene in yields exceeding about 98% while simultaneously obtaining a product having a hexachlorobenzene content of not more than about 0.6%.
We have now found that there is a critical relationship between hexachlorobenzene formation and the precise addition and post-addition reaction parameters utilized; and further that, in order to meet the requirements set forth above, several conditions must be met. A low initial reaction temperature during the addition must be utilized. Following the low temperature addition period the resulting reaction mixture must be heated to an intermediate temperature range to complete the reaction and must then be held in this range for a period of time sufficient to deplete the nitric acid concentration to a predetermined value. Thereafter the mixture may be heated above the melt temperature and then cooled to recrystallize pentachloronitrobenzene in high yields, having excellent crystal characteristics and having a low hexachlorobenzene content.